For example, in recent years, in soda electrolysis, an ion-exchange membrane alkali chloride electrolytic cell for producing high-purity alkali metal hydroxide with high current efficiency and at a low voltage, particularly, a filter-press-type zero-gap electrolytic cell or a finite electrolytic cell in which an anode for electrolysis and a cathode for electrolysis come into contact with each other with an ion-exchange membrane interposed therebetween has been generally used.
When the cathode for electrolysis is used as a cathode for the ion-exchange membrane electrolysis, particularly, a cathode for a finite electrolytic cell or a zero-gap electrolytic cell, a conductive cathode base having a plurality of intersection portions between members (hereinafter, simply referred to as a “conductive cathode base having a plurality of intersection portions” or a “wire mesh-like cathode base”), such as a wire mesh, expanded metal, perforated metal, or equivalents thereto, is used as a conductive cathode base of the cathode for electrolysis. In general, a cathode catalyst layer is positively formed on one surface of the conductive cathode base having a plurality of intersection portions by an application method to manufacture a cathode, and the cathode is used with the side (cathode catalyst layer side) on which the cathode catalyst layer is formed coming into contact with one surface of the ion-exchange membrane or with a very small space therebetween. The anode for electrolysis comes into contact with a surface opposite to the ion-exchange membrane or a very small space is interposed therebetween.
In this type of electrolytic cell, in general, the base of the cathode is made of nickel or nickel alloy. The conductive cathode base having a plurality of intersection portions is used as the base of the cathode. A cathode catalyst layer including a cathode catalyst component which is expensive rare metal, such as platinum, is formed on at least one surface of the base.
For example, Patent Literature 1 discloses a method for manufacturing a cathode for electrolysis which is used in a zero-gap electrolytic cell in which an anode and a cathode come into contact with each other with an ion-exchange membrane interposed therebetween. In the manufacturing method, the following are described: the thickness of a conductive base having a plurality of intersection portions when the conductive base is used in the anode and the cathode; an aperture ratio; the thickness of a cathode catalyst layer; the thickness of an uneven portion in the surface of the cathode; and preprocessings such as a baking process, a shaping process, a planarizing process using rolling, a roughening process using a blast, a cleaning process with an acid, an etching process, and a process of improving corrosion resistance.
As disclosed in Patent Literature 1, in the related art, preprocessings, such as a baking process, a shaping process, a planarizing process using rolling, a roughening process using a blast, a cleaning process with an acid, an etching process, and a process of improving corrosion resistance, is performed for the conductive base having a plurality of intersection portions. Then, a cathode catalyst layer including a cathode catalyst component which is made of platinum-group metal and/or an oxide thereof is formed on one surface of the conductive base.
A step of forming the cathode catalyst layer is called an activation step. In the step, in general, an application liquid including a starting material which will be a cathode catalyst component (hereinafter, simply referred to as a starting material) is applied to the base and the application liquid applied to the base is dried and baked. Specifically, in the activation step, in general, first, an application liquid in which a starting material is dissolved is prepared. The application liquid is applied to one surface of the conductive base having a plurality of holes which has been subjected to preprocessing. Then, the application liquid is dried and baked to form a cathode catalyst layer. At that time, in order to forma desired cathode catalyst layer, the step of applying the application liquid and drying and baking the application liquid applied to the base is repeated a plurality of times until a desired amount of cathode catalyst component is attached to the front surface of the conductive cathode base. A cathode catalyst layer including a cathode catalyst component (a material forming a catalyst layer) which is expensive rare metal, such as platinum, is formed through the application, dry, and baking steps. The application step of applying the application liquid to the base is generally performed by methods using a roller and a spray, a brush application method, an electrostatic coating method, and other methods. In addition, in the baking step, heating is generally performed by, for example, an electric furnace.